In EDM, electric energy is supplied in the form of discrete electrical pulses across the machining gap filled with a machining fluid or liquid dielectric (e.g., kerosine, transformer oil, distilled water or weakly conductive water) to effect a succession of electrical discharages between the tool electrode and the workpiece to remove material from the latter. As material removal proceeds, the tool electrode is advanced relatively toward the workpiece by servo feed means adapted to maintain the machining gap spacing substantially constant thereby to allow material-removal discharges to be successively created. The contamination of the machining gap region with chips, tar and gases produced by machining discharges may be removed by continuously or intermittently flushing the gap with the fresh machining fluid and/or intermittently or cyclically retracting the tool electrode away from the workpiece to allow the fresh machining medium to be pumped into the machining gap and the machining contaminants to be carried away from the latter.
Parameters of individual and successive electrical discharges, i.e. pulse on-time Ton, peak current Ip and off-time Toff, are determinative of machining results, e.g. removal rate, surface roughness and relative electrode wear and, therefore, are individually or in combination particularly adjusted to establish a particular machining condition suitable to achieve desired machining results.
As proposed by the inventor in Japanese Patent Specifications No. 39-20494 published Sept. 19, 1964 and No. 44-8317 published Apr. 18, 1969, there is known an improved EDM pulse supply technique in which a succession of pulses trains individually consisting of elementary pulses of predetermined on-time .tau.on and off-time .tau.off are applied across the machining gap, the pulse trains having a duration Ton with the successive trains being separated by a cut-off time interval Toff. Elementary pulses in each train may be modified as to its triggering or peak voltage v as described in the aforementioned Japanese Patent Specification No. 44-8317. Others have also proposed various circuits generally of this or similar genre, which are described, for example, in U.S. Pat. No. 3,056,065 and No. 3,943,321.
Fine-surface and precision machining results are obtainable with a train of elementary pulses of a duration .tau.on set to be short and preferably at a minimum which, when repeated at an elevated frequency or with a pulse interval .tau.off set to be at most equal in length to the pulse duration .tau.on, permits an increased removal rate to be obtained. The shortness of the pulse interval .tau.off may, however, cause the production of machining chips and other products to being about a continuous arc discharge with ease within a short time period. This possibility can advantageously be eliminated or alleviated by the presence of cutoff time periods Toff which separate from one another the successive trains Ton of elementary pulses (.tau.on, .tau.off). Thus, the cyclic interruption of elementary pulses allows accumulated machining products to be carried away within each interruption period from the machining gap and may thereby serve to maintain the latter substantially free from continued contamination. By establishing the cut-off interval Toff at a level sufficient to allow clearing of the contaminants resulting from the machining action of the previous train of elementary pulses, the gap can be ready to accept the next train of elementary pulses to continue stabilized machining discharges.
On the other hand, in the interest of increasing the removal rate, it is desirable to have the electrode-feed servo system operate so as to minimize production of non-striking pulses or pulses without discharge. In order to facilitate production of discharges under application of successive trains of elementary pulses, the gap can be reduced but this can also facilitate production of a continuous arc or result in difficulty in gap flushing or decontamination. The attempt to facilitate machining discharges by reducing the number of elementary pulses in each train causes a drop in removal rate while the setting of the servo system so as to widen the threshold gap spacing leads to an increased frequency of non-striking pulses.